Micro mirrors made by micro-electro-mechanical system (MEMS) process are widely used for light beam scanning devices, such as a scanning mirror in a mini projector. Conventionally, it is driven by electrostatic forces at high rotating speed.
Please refer to FIG. 1A. U.S. Pat. No. 6,817,725 discloses a micro mirror unit 100 for incorporation in a device, such as an optical switch. The micro mirror unit 100 includes a mirror-formed portion 110 having an upper surface provided with a mirror surface (not illustrated), an inner frame 120 and an outer frame 120 (partly un-illustrated), each formed with comb-like electrodes integrally therewith. Specifically, the mirror-formed portion 110 has ends facing away from each other, and a pair of comb-like electrodes 110a and 110b are formed respectively on these ends. In the inner frame 120, a pair of comb-like electrodes 120a and 120b extend inwardly, corresponding to the comb-like electrodes 110a and 110b. Also, a pair of comb-like electrodes 120c and 120d extend outwardly. In the outer frame 120, a pair of comb-like electrodes 120a and 120b extend inwardly, corresponding to the comb-like electrodes 120c and 120d. The mirror-formed portion 110 and the inner frame 120 are connected with each other by a pair of torsion bars 140. The inner frame 120 and the outer frame 120 are connected with each other by a pair of torsion bars 150. The pair of torsion bars 140 provides a rotation axis for the mirror-formed portion 110 to rotate with respect to the inner frame 120. The pair of torsion bars 150 provides a rotation axis for the inner frame 120, as well as for the associating mirror-formed portion 110, to rotate with respect to the outer frame 120.
With the above arrangement, in the micro mirror unit 100, a pair of comb-like electrodes, such as the comb-like electrode 110a and the comb-like electrode 120a, are opposed closely to each other for generation of static electric force, and take positions as shown in FIG. 1B, i.e. one of the electrode assuming a lower position and the other assuming an upper position, when there is no voltage applied. When an electric voltage is applied, as shown in FIG. 1C, the comb-like electrode 110a is drawn toward the comb-like electrode 120a, thereby rotating the mirror-formed portion 110. More specifically, in FIG. 1A, when the comb-like electrode 110a is given a positive charge whereas the comb-like electrode 120a is given a negative charge, the mirror-formed portion 110 is rotated in a direction M1 while twisting the pair of torsion bars 140. On the other hand, when the comb-like electrode 120c is given a positive charge whereas the comb-like electrode 120a is given a negative charge, the inner frame 120 is rotated in a direction M2 while twisting the pair of torsion bars 150.
As a conventional method, the micro mirror unit 100 can be made from an SOI (Silicon on Insulator) wafer which sandwiches an insulating layer between silicon layers. However, according to the conventional method of manufacture as described above, the thickness of the wafer is directly dependent on the thickness of the micro mirror unit 100. Specifically, the thickness of the micro mirror unit 100 is identical with the thickness of the wafer which is used for the formation of the micro mirror unit. For this reason, according to the conventional method, the material wafer must have the same thickness as the thickness of the micro mirror unit 100 to be manufactured. This means that if the micro mirror unit 100 is to be thin, the wafer of the same thinness must be used. For example, take a case of manufacturing a micro mirror unit 100 having a mirror surface having a size of about 100 through 725 μm. In view of a mass of the entire moving part including the mirror-formed portion 110 and the inner frame 120, the amount of movement of the moving part, the size of the comb-like electrodes necessary for achieving the amount of movement, etc considered comprehensively, a desirable thickness of the moving part or the micro mirror unit 100 is determined. In this particular case, the desirable thickness is 100 through 200 μm. As a result, in order to manufacture the micro mirror unit 100 having such a thickness, a wafer having the thickness of 100 through 200 μm is used.
According to the conventional method, in order to manufacture a thin micro mirror unit 100, a correspondingly thin wafer must be used. This means that the greater diameter the wafer has, the more difficult to handle the wafer. For instance, take a case in which a micro mirror unit 100 is to be manufactured from an SOI wafer having a thickness of 200 μm and a diameter of 6 inches. Often, the wafer is broken in a midway of the manufacturing process. Further, the limitation on the size of the flat surface of the wafer places a limitation on the manufacture of micro mirror array chips. Specifically, when the micro mirror array chips are manufactured by forming a plurality of micro mirror units in an array pattern on a single substrate, the size of the array is limited.
Precise lateral alignment between two sets of comb-like electrodes, e.g., electrodes 120c and 120a, are inherently difficult to achieve since they are not coplanar and are fabricated in two different layers of the substrate. This can further result in non-linear and unstable behavior. Furthermore, driving force provided by the comb-like electrodes is limited and power needed by the electrodes to drive the mirror is large.
Please refer to FIG. 2A. In order to overcome the aforementioned disadvantages, magnets 210 are used to replace the comb-like electrodes for providing driving force to rotate a biaxial mirror assembly 200. Two side magnets 210a with the same magnetization direction are positioned on both sides of the biaxial mirror assembly 200 above a bottom magnet 210b with an opposite magnetization direction. However, these two side magnets 210a occupy a large space, and thus, such a structure is too big. Furthermore, it is hard to increase the magnetic field without drastically increasing the magnet volume.
To minimize the total size another structure having two magnets 230a placed on top of a biaxial mirror assembly 220, and another two magnets 230b placed below the biaxial mirror assembly 220 is shown in FIG. 2B. However, the total size of such structure is still too large due to the fact that the magnets 230a and 230b need to allow enough space for the biaxial mirror assembly 220 to rotate.
Therefore, a biaxial mirror assembly having a small size with large driving force is desperately desired.